This invention relates to an apparatus to calculate remaining capacity, which detects battery current and calculates remaining battery capacity.
Remaining battery capacity can be calculated by subtracting discharge capacity from charge capacity. Charge capacity is calculated by integrating charging current, and discharge capacity is calculated by integrating discharge current. Consequently, battery charging current and discharge current are detected in order to calculate remaining battery capacity. A prior art apparatus calculates remaining capacity by detecting battery current via a current detection section, and calculating remaining capacity by integrating the detected current values.
Patent Reference [1]: Japanese Non-Examined Patent Publication No. 274670/1998
In prior art apparatus, which calculates remaining capacity by integrating current detected by a current detection section, current detection section errors cause remaining capacity values to go astray. These errors accumulate over time and become large. When battery capacity cannot be accurately calculated in a system which controls charging and discharging based on remaining capacity, remaining capacity errors are a cause of battery over-charging, battery over-discharge, and battery degradation. This drawback can be prevented by accurate current detection. However, if current is detected with high precision, the cost of components becomes extremely high. For example, a remaining capacity computation section, which calculates remaining capacity by operating on a digital signal, converts an analog signal from the current detection section to a digital signal via an analog to digital (A/D) converter. However, noise is generated by digitizing, or converting an analog signal to discrete digital values, causing degradation of current measurement resolution. For example, the resolution of an 8-bit A/D converter is 1/256. Using an 8-bit A/D converter with a maximum detected current of 2.56A, corresponds to a resolution of 10 mA for each bit of the A/D converter. Measured current for the A/D converter is at 10 mA intervals, or 10 mA, 20 mA, 30 mA, 40 mA, etc. Therefore, when actual discharge current is 16 mA, for example, the current detection section detects 20 mA of discharge current. Consequently, a large 25% error is generated. This error accumulates leading the remaining capacity astray while degrading the precision of remaining capacity calculations. Errors resulting from digitizing can be reduced by increasing the number of bits of the A/D converter. For example, using a 10-bit A/D converter can reduce digitizing noise by ¼ that of an 8-bit A/D converter. However, A/D converters have the drawback that the greater the number of bits, the higher the cost. Further, even if the output of the current detection section is digitized using a 10-bit or greater A/D converter, there is also some inherent measurement error in the current detection section itself, and that error is a cause of inaccuracies in remaining capacity calculation as well.